The reaction of polyether polyols and/or polyester polyols in a prepolymer process or a one-shot process with polyisocyanates, chain-lengthening agents and, optionally, auxiliary agents, such as catalysts, stabilizers and blowing agents, to produce polyurethane foams, elastomers, lacquers and adhesives is known. Common to all these synthetic resins is that they contain polyether and/or polyester polyol residues as "soft segments" and urethane and/or urea groups (as obtained by reacting hydroxyl groups of polyether or polyester polyol and, in particular, the hydroxyl and/or amino groups of chain-lengthening agents with polyisocyanates) as "hard segments" coupled as bridging members between these soft segments. The overall properties of such a polymer, and in particular its response to high temperatures and its hardness and long term flexibility, depend to a large extent on the number and properties of these hard segments. As the molecular weight of the polyether or polyester polyol increases, given otherwise identical conditions, the mechanical properties of the resin markedly deteriorate, particularly the tensile strength and structural stability. This deterioration may only be compensated to a very slight extent by increasing the number of hard segments (addition of a larger quantity of chain-lengthening agent). Conversely, a reduction in the quantity of diisocyanates or chain-lengthening agents in such systems (for a given molecular weight of polyurethane) results in a drastic deterioration of all the mechanical properties. The overall properties are therefore worse when a polymer is built up of equimolar quantities of higher molecular weight polyol and polyisocyanate (i.e. without the addition of low molecular weight chain-lengthening agents). Under these conditions, no successful outcome is obtained even if the high molecular weight polyols are linked through other bridging members, such as urea, aromatic ester, amide, imino, sulfone or hydantoin groups.